FileUtils: Replace FileNameList with an alias

The functionality in that class is covered by initializers and
Utils::unique, so there is no need to keep a custom class.

This way FileNameList plays way nicer with utils/algorithm.h.

Change-Id: I8eeae6aca0558ecd998920c4fdfc5ea56bf75501
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/qmakeprojectmanager/qmakenodes.cpp

@@ -2427,8 +2427,7 @@ FileNameList QmakeProFileNode::subDirsPaths(QtSupport::ProFileReader *reader,
         }
     }
 
-    subProjectPaths.removeDuplicates();
-    return subProjectPaths;
+    return Utils::filteredUnique(subProjectPaths);
 }
 
 TargetInformation QmakeProFileNode::targetInformation(QtSupport::ProFileReader *reader, QtSupport::ProFileReader *readerBuildPass, const QString &buildDir, const QString &projectFilePath)